 html,body{height: 100%}     /*这里很关键*/
 
        .outer-wrap{
            /*只有同时为html和body设置height: 100%时，这里的height才生效，
            并且随浏览器窗口变化始终保持和浏览器视窗等高*/
            height: 100%;    
            position: relative;
          
        }
        .login-panel{
            width: 400px;
            height: 250px;
            position: absolute;
            top: 50%;
            left: 50%;
            margin-top: -150px;
            margin-left: -200px;
        }
        .a{
        	
        	height: 30px;
        	width: 200px;
        }
        .b{
        	margin-top: 20px;
        	width: 200px;
        	height: 30px;
        }
		.c{
			width: 120px;
			height: 50px;		
			margin-left: 70px;
			margin-top: 20px;	
			background: cornflowerblue;
			padding: 10px 20px;
			font-size: 16px;
			text-align: center;
			display: inline-block;
		}
		.c:hover{
			box-shadow: 0 12px 16px 0 rgba(0,0,0,0.24),0 17px 50px 0 rgba(0,0,0,0.19)
		}
		.d{
			border-radius: 80%; 
			margin-left: 70px;
		}
		